6
$\begingroup$

It is a known fact that if $X$ is a skew-symmetric matrix, then $e^X$ is an orthogonal matrix.

Is it also true the opposite, ie that any orthogonal matrix admits a representation like $e^X$ for some $X$ skew-symmetric?

If not, is there a way to parametrize the space of the orthogonal matrices? With this I mean, a function $f$ that takes some parameters $\Theta$ and returns a matrix $O$ such that:

  • $f(\Theta)=O$ is orthogonal
  • If $O$ is orthogonal, there exists a set of parameters $\Theta$ s.t. $f(\Theta)=O$?

Does the topic get easier if we focus on orthonormal matrices?

$\endgroup$
1
  • 1
    $\begingroup$ "Orthonormal matrix" is not a common term. "Orthogonal matrix" already means a matrix with orthonormal rows / columns. $\endgroup$ Commented Jul 24, 2017 at 18:01

3 Answers 3

8
$\begingroup$

As you said, if $X$ is a skew-symmetric matrix then $e^X$ is orthogonal, and precisely $e^X$ is a special orthogonal matrix, i.e. with determinant equal to $+1$: $$\det(e^X)=e^{\operatorname{Tr}(X)}=e^0=1$$ Conversely, for $O\in SO_n(\Bbb R)$, there is an orthonormal basis in which $O$ is similar to $$S=\operatorname{diag}(I_p,R(\theta_1),\ldots,R(\theta_s))$$ where $$R(\theta_i)=\begin{pmatrix}\cos\theta_i&-\sin\theta_i\\\sin\theta_i&\cos\theta_i\end{pmatrix}$$ Moreover, we have $\exp(J_i)=R(\theta_i)$ with $$J_i=\begin{pmatrix}0&-\theta_i\\\theta_i&0\end{pmatrix}$$ so we define the skew-symmetric matrix $$A=\operatorname{diag}(0_p, J_1,\ldots,J_s)$$ and we get $e^A=S$.

$\endgroup$
2
  • $\begingroup$ however, this method only parametrize special orthogonal matrix; is there any similar way of parameterize orthogonal matrix with determinant -1 as well ? $\endgroup$
    – ElleryL
    Commented Dec 11, 2018 at 19:20
  • 1
    $\begingroup$ The orthogonal matrices with determinant -1 are disconnected from those with determinant 1, so there is no continuous parametrization that covers both. However, if you just want to parametrize those with determinant -1, then you can take any parametrization of those with determinant 1 and then negate an arbitrary row or column. $\endgroup$ Commented Jan 17, 2021 at 16:32
7
$\begingroup$

Every orthogonal matrix with determinant $1$ has the form $\exp(X)$ with $X$ skew-symmetric. Another representation of orthogonal matrices is the Cayley parameterisation: $(I+X)(I-X)^{-1}$ is orthogonal whenever $X$ is skew-symmetric. This produces all orthogonal matrices of determinant $1$ which do not have $-1$ as an eigenvalue.

$\endgroup$
1
  • $\begingroup$ however, this method only parametrize special orthogonal matrix; is there any similar way of parameterize orthogonal matrix with determinant -1 as well $\endgroup$
    – ElleryL
    Commented Dec 11, 2018 at 19:23
2
$\begingroup$

As Lord Shark of the Unknown already told, the answer is negative for $O(n,\mathbb{R})$, but it is affirmative for $SO(n,\mathbb{R})$. That the answer is negative in the case of $O(n,\mathbb{R})$ follows from the fact that it is not connected, whereas the space of all skew-symmetric matrices is (together with the fact that the exponential map is continuous).

Actually, if $K$ is a compact and connected matrix group and if$$\mathfrak{k}=\left\{X\in M(n,\mathbb{R})\,|\,(\forall t\in\mathbb{R}):\exp(tX)\in K\right\},$$then $\exp(\mathfrak{k})=K$.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .